In the production of color photographic images, it is usually necessary to remove the silver image which is formed coincident with the dye image. This can be done by oxidizing the silver by means of a suitable oxidizing agent, commonly referred to as a bleaching agent, in the presence of halide ion, followed by dissolving the silver halide so formed in a silver halide solvent, commonly referred to as a fixing agent. Alternatively, the bleaching agent and fixing agent can be combined in a bleach-fixing solution and the silver removed in one step by use of such solution.
A wide variety of bleaching agents are known for use in photographic processing, for example, ferricyanide bleaching agents, persulfate bleaching agents, dichromate bleaching agents, permanganate bleaching agents, ferric chloride, and water-soluble quinones.
It is particularly well known to use a ferric complex of an aminopolycarboxylic acid as a bleaching agent in photographic color processing. Such complexes are used in both bleaching compositions and bleach-fixing compositions. A very large number of different compounds of the aminopolycarboxylic acid class are disclosed in the prior art as being useful photographic bleaching agents. However, the usual commercial practice is to use an ammonium or alkali metal salt of a ferric complex of ethylenediaminetetraacetic acid (EDTA) or of a ferric complex of propylenediaminetetraacetic acid (PDTA). In addition to such ferric complex salt, a bleach-fix solution typically contains a thiosulfate fixing agent and a sulfite that functions as a preservative agent that prevents the thiosulfate from breaking down.
Among the numerous patents describing the use of ferric complexes of aminopolycarboxylic acids in bleaching and/or bleach-fixing baths are U.S. Pat. Nos. 3,241,966, 3,615,508 and 3,767,401 and British patents 1,365,453, 1,392,163, and 1,394,357.
For both economic and ecological reasons, it is common practice in the trade to regenerate spent bleach-fix solutions to permit their reuse. The spent solutions are high in iron, biological oxygen demand (BOD) and chemical oxygen demand (COD) and regeneration greatly reduces the problem of disposing of such effluent. As part of such regeneration, it is necessary to remove the silver from the spent bleach-fix, and this is most advantageously accomplished by the use of electrolytic silver recovery techniques. Such techniques are very well known in the photographic art, and methods of electrolytic silver recovery and equipment for use therein are described in the patent literature, for example, in Seiler et al, U.S. Pat. No. 4,014,764, in Baden et al, U.S. Pat. No. 4,036,715 and in Blake et al, U.S. Pat. No. 4,211,630.
The electrolytic recovery of silver from a spent bleach-fix solution requires careful control of the pH of the solution. For optimum results, the process should be carried out at a pH of above 7 and preferably at a pH of above 7.5. One reason for this is that the reduction potential of iron (III) is pH dependent and, at low pH, the iron is more easily reduced. The pH also affects the sulfite-bisulfite equilibrium. At low pH, such as a pH of below 7, the current efficiency for silver reduction is relatively low, due to the competing reactions of iron (III) and bisulfite at the cathode. Thus, to obtain efficient electrolytic recovery of silver from a bleach-fix, the pH should be above 7 and preferably above 7.5. Above pH 7, raising the pH increases the recovery rate and current efficiency; while below pH 7, the recovery rate and current efficiency are low and independent of pH. The upper limit of pH for the electrolytic silver recovery process is determined by the onset of iron precipitation or, if an ammonium salt of the ferric complex has been used, by ammonia evolution. Typically, very good results are obtained over a pH range of from about 7.5 to about 9.
While electrolytic silver recovery from a bleach-fix solution should be carried out at a pH of above 7, as explained above, the pH of a working strength bleach-fix solution should be below 7--and preferably in the range of from about 5 to about 6.5--in order to provide a highly active bleach-fix and thus a short bleach-fixing time such as a time of one minute or less.
Thus, to effectively regenerate a highly active bleach-fix solution using electrolytic methods of silver recovery, it is necessary to increase the pH of the spent bleach-fixing solution prior to the electrolytic process, and then decrease it to an optimum level for a working strength bleach-fix solution after the electrolytic process. This can be done by the addition of a basic agent before electrolytic silver recovery and the addition of an acidic agent after electrolytic silver recovery, but this complicates the process and, when strong acids and bases are used for this purpose, necessitates the storage and handling of such potentially hazardous materials.
In the form in which they are typically employed, photographic processing solutions are dilute aqueous solutions and thus it is not generally feasible, from an economic standpoint, to package, transport, and store processing solutions of working strength, since this would involve the packaging, transporting, and storage of large amounts of water. This problem is generally avoided by packaging, transport and storage of photographic processing solutions in the form of liquid concentrates, that is, concentrated solutions which merely have to be diluted with water to obtain a working strength solution. With many photographic processing compositions, there is a serious problem of unwanted chemical interactions which take place between certain of the components, and this frequently prevents the compounding of all of the components in a single liquid concentrate, and necessitates the separation of the components into two or more parts which are subsequently combined to form the working solution. Often, as many as three or four different parts are required, and these parts constitute what is referred to in the trade as a photographic "processing kit".
It is toward the objective of providing a processing kit for use in the regeneration of spent photographic bleach-fixing solutions, employing an iron complex of an aminopolycarboxylic acid, that the present invention is directed. A more specific objective is to provide such a kit and a method of regeneration that will enable both bleach-fixing and electrolytic silver recovery to be carried out under optimum conditions.